Tetracycline staining of developing teeth has a well-known incidence. It is used clinically to treat acute infections or acne. Avoidance of the agent during tooth development years is relatively easy since many other agents are available. It was believed that the use of tetracycline for treatment of acne during the post tooth development years was safe. However, very recent evidence suggests that the drug is capable of causing stained dentin during these later years, probably by diffusing from the circulation into dentin tubules via dentinal fluid. The qualitative binding of tetracycline to tooth structure has been reported by several investigators. However, quantitative studies of this process are lacking. We propose to study the quantitative binding of 3H-tetracycline to dentin in vitro, with the use of extracted human third molar dentin discs. The study will include binding by diffusion or under influence of pressure equal to pulpal tissue pressure. In addition, we will study binding with and without the presence of a smear layer, two examples of the in vivo situation. The study will include the influence of dentin thickness on binding. Binding of tetracycline will be compared to iodide (125I) an agent that is known not to bind to dentin tubules. Eventually the ultimate goal is to attempt to establish guidelines for tetracycline use in the teen years and early twenties. Also, perhaps a chemical can be found that will either reverse tetracycline binding or prevent or delay binding prior to its administration.